Apple to open iPhone to outside developers:
My hope is that Skype will be able to run from an iPhone, allowing
Skype access from wi-fi spots. This would mean that you wouldn’t be
using your ATT minutes while at hot spots or at your home wifi as you
would be making calls through Skype’s network instead. For now, here is the workaround:
http://blog.tmcnet.com/blog/tom-keating/skype/skype-on-iphone.asp
This software lets you use Skype on an iPhone or a Windows Mobile Phone (like Tilt):
http://www.shapeservices.com/en/products/details.php?product=skype&platform=none